With outstanding advancement of electronics technology in recent years, portable electronic devices have been made smaller, lighter, and thinner, and equipped with multiple functions. According to this, batteries as power sources for electronic devices are required to be smaller, lighter, thinner, and highly reliable. In response to the demand, there has been proposed a multilayer lithium ion secondary battery in which a plurality of positive layers and a plurality of negative layers are alternately laminated with solid electrolyte layers interposed therebetween. The multilayer lithium ion secondary battery is assembled by laminating battery cells with a thickness of several tens of micrometers. Therefore, the battery can be readily made smaller, lighter, and thinner. In particular, a parallel or series-parallel laminated battery is excellent in achieving a large discharge capacity with a small cell area. In addition, because an all-solid lithium ion secondary battery includes solid electrolyte instead of electrolytic solution, the all-solid lithium ion secondary battery is immune to leakage or depletion of liquid and has high reliability. Furthermore, because the all-solid lithium ion secondary battery includes lithium, the all-solid lithium ion secondary battery provides high voltage and high energy density.
FIG. 8 is a cross sectional view illustrating a conventional lithium ion secondary battery (Patent Document 1). The conventional lithium ion secondary battery is configured to have a laminated body in which a positive layer 101, a solid electrolyte layer 102, and a negative layer 103 are laminated in sequence; and terminal electrodes 104 and 105 connected electrically to the positive layer 101 and the negative layer 103, respectively. FIG. 8 shows the battery formed by one laminated body for convenience of description. In actuality, however, the battery is generally formed by laminating the large number of positive layers, solid electrolyte layers, and negative layers in sequence to provide a large battery capacity. An active material constituting the positive layers is different from an active material constituting the negative layers. That is, a substance with a higher oxidation-reduction potential is selected as a positive active material, and a substance with a lower oxidation-reduction potential is selected as a negative active material. In the thus structured battery, if the terminal electrode on the negative side is regarded to be under a reference voltage, a positive voltage is applied to the terminal electrode on the positive side to charge the battery. Meanwhile, on discharging, a positive voltage is output from the terminal electrode on the positive side.
In contrast, since the positive active material has only the function as a positive material, and the negative active material has only the function as a negative material, the battery is not charged when a negative voltage is applied to the positive side terminal electrode and a positive voltage is applied to the negative side terminal electrode (so-called “reverse voltage application”).
In the conventional secondary battery, each active material layer is clearly determined as a positive electrode or a negative electrode (polar battery). Thus, when reverse voltage is applied to the conventional secondary battery, the battery does not function normally as a secondary battery. Furthermore, reverse voltage should not be applied to the secondary battery, particularly when a liquid electrolyte is employed. Otherwise, the electrode metal may be eluted into the electrolyte and precipitated. The precipitated metal may penetrate the separator, and a peeled metal may float in the liquid electrolyte. As a result, the battery may be broken by short circuit and heating in the battery.
Even when a solid electrolyte is used as the electrolyte, the active material could be damaged by reverse charging, whereby the original function of the secondary battery may not be recovered upon recharging.
For these reasons, the conventional secondary batteries have been strictly designed so that a high voltage can be applied from a negative material to a positive active material at the time of charging. Further, in the case of forming an assembled battery in which polar secondary batteries are connected in series, the positive electrode of one cell is electrically connected to the negative electrode of another cell without fail. Thus, the product of the electromotive force of a single cell and the number of the cells connected in series is obtained as an output voltage. The output voltage of the assembled battery is therefore uniquely determined, and has been impossible to convert the output voltage without using an external electronic circuit.